Section 6
SEC. 6. All material, supplies, or articles of whatever kind and nature, imported and passed free of duty under the provisions of this Act which shall not be used in the construction or equipment of a line of railroad, and all used construction machinery and equipment above that which can be reasonably used in the maintenance and operation of the road shall be inventoried and assessed for duty upon the completion of the construction and equipment of the line of railroad for which such material, supplies, articles, or machinery were imported : Provided, That used construction machinery and equipment shall be assessed for duty at a valuation to he fixed thereon by the Insular Collector of Customs.